"Honest question. Why don't people buy old laptops instead of cheaper new ones?"

That's exactly how I've done desktop Linux: buy ex-corporate laptops from a reputable refurbisher. You get a quality machine with a good chassis and keyboard, highly-compatible internals (all-Intel) and a decent screen for a good price. Performance doesn't matter if you aren't doing stuff that actually needs the power. Trade-offs are worse power management, and short life-span (I change my Linux laptop every couple of years).